Silicon carbide resistance igniter

ABSTRACT

A monolithic ceramic resistance igniter of simple configuration is composed essentially of polycrystalline silicon carbide adapted for use in gas and liquid fuel burning systems. As a result of the combination of its sintered silicon carbide composition, its microstructure, controlled density and large cross-sectional area, the igniter possesses an unusually high degree of physical ruggedness. The igniter will attain a temperature of about 1,000*C in well under 20 seconds drawing a maximum of 6 amps at 132 volts, with a room temperature resistivity of 0.10 to 1.70 ohm centimeters and a resistivity at about 1000*C of from 0.06 to 0.26 ohm centimeter. The igniter also has a physical construction such that a high percentage of its hot surface area radiates directly to the environment.

United States Patent 1191 Fredriksson et al.

1 SILICON CARBIDE RESISTANCE IGNITER [75] Inventors: John I.Fredriksson, Holden; Samuel H. Coes. Northboro, both of Mass.

[51] Int. Cl. F23q 7/10 [58] Field of Search 317/79, 80, 81. 98;219/264, 270, 552, 553; 338/22. 262. 330;

[ Apr. 1, 1975 3.454.345 7/1969 Dyre 431/66 3.467.312 9/1969 Terrell317/93 x 3.502.419 3/1970 Perl...; 431/66 3.597.139 8/1971 Elders 431/663.681.737 8/1972 Magnussoti 61 219/553 x Primary Examiner-volodymyr Y.Mayewsky Attorney. Agent, or Firm-Arthur A. Loiselle, Jr.

[57] ABSTRACT A monolithic ceramic resistance igniter of simpleconfiguration is composed essentially of polycrystalline silicon carbideadapted for use in gas and liquid fuel burning systems. As a result ofthe combination of its sintered silicon carbide composition, itsmicrostructure, controlled density and lar e cross-sectional area, theigniter possesses an unusual y high degree of physical ruggedness. Theigniter will attain a temperature of about 1,000C in well under 20seconds drawing a maximum of 6 amps at 132 volts, with a roomtemperature resistivity of 0. 10 to 1.70 ohm centimeters and aresistivity at about 1000C of from 0.06 to 0.26 ohm centimeter. Theigniter also has a physical construction such that a high percentage ofits hot surface area 12 Claims, 3 Drawing Figures [56] References CitedUNITED STATES PATENTS 1.906.963 5/1933 Hcyroth 338/330 2.001.297 5/1935Boyles 338/330 2.735.881 2/1956 Mann..... 13/25 2.933.896 4/1960 Ferrie431/262 X 3.282.324 11/1966 Romanelli 317/98 x 'adlates d'rwly theenv'ronmen" 3372.305 3/1968 MlkUICC 317/98 SILICON CARBIDE RESISTANCEIGNITER BACKGROUND OF THE INVENTION The invention relates to ignitersfor fuel burning devices such as domestic and industrial liquid fuel andgas burning appliances. More particularly. the invention relates toceramic resistance igniters for gas burning appliances such as kitchenranges, furnaces, clothes dryers and the like.

The concept of non-pilot light igniters has been known for years. Theearlier type of igniter was the incandescent wire device such as anelectrically heated platinum wire coil. These are fragile and, in mostapplications, require a step-down transformer. Ceramic resistanceigniters made their appearance in about l937. U.S. Pat. No. 2,089,394describes a total electrical ignition system in which a ceramicresistance igniter composed of Durhy Material" is utilized to ignite afluid fuel system. Durhy is a dense sintered silicon carbide impregnatedwith silicon. A U-shaped ceramic igniter is disclosed in U.S. Pat. No.2,095,253 where the igniter is composed of sintered and siliconimpregnated silicon carbide. This igniter element is formed by firstperforming l grit (142 microns) and finer silicon carbide material, intorods of suitable length, which are then fired to presinter the siliconcarbide. The rods are then cut into the desired length and slotted toform a U-shaped element which is subsequently impregnated with siliconmetal. Another basic type of silicon carbide igniter is that describedin U.S. Pat. No. 3,052,8l4. This is a sparkplug type igniter as opposedto the pure resistance type mentioned above and is composed of siliconnitride bonded with silicon carbide. Still another silicon carbideigniter device is described in U.S. Pat. No. 3,282,324 as part of acomplete ignition and heat injection system. ln this case the siliconcarbide is a sintered silicon carbide cylinder having a spiral cut whichprovides a relatively small percentage of the hot area which radiatesdirectly to the environment.

By nature of their use, resistance igniters must be small in dimension,particularly in terms of their crosssection and overall length. Becauseof these physical parameter restrictions, prior art silicon carbideigniters are very fragile. As a result, attempts have been made tophysically reinforce ceramic resistance igniters by such approaches asthat described in U.S. Pat. Nos. 3,372,305 and 3,467,8l2. Both of theseigniters have a spiral configuration which is fabricated of a sinteredtube of silicon carbide which is made as dense as possible. The spiralconfiguration is cut in the sintered silicon carbide tube, which is thensupported by an aluminum oxide rod which passes through the opening ofthe spiral igniter body.

Still another type of resistance igniter is described in US. Pat. No.3,454,345. This igniter is composed ofa sintered mixture of siliconcarbide and silicon oxynitride wherein the silicon oxynitride functionsas a bond for a relatively coarse lOF silicon carbide, i.e., a mixtureof particles of 1,340 microns and finer in size with 10 percent byweight of silicon oxynitride. This silicon carbide/silicon oxynitridemixture is one manufactured and sold by the Norton Company, Worcester,Mass, and its foreign affiliates under the trademark CRYS- TOLON 63.

Despite the substantial amount of activity in the ceramic resistanceigniter field, the igniters enjoying most widespread use today, for mostapplications, are still of the pilot light type. In view of the currentenergy crisis and the result of various surveys which show that pilotlights consume from l0 to 15 percent of the total gas consumed in thiscountry, there is obviously a compelling need for an igniter to replacethe presently used pilot light.

It is, therefore, a principal object of the present invention to providea ceramic resistance igniter for liquid and gas fuel burning deviceswhich is free of the foregoing deficiencies, and which is physicallyrugged, heats rapidly, survives hundreds of thousands of heating cycles,is simple electrically and structurally, has low susceptibility topremature burn out, and radiates primarily to the environment.

SUMMARY OF THE INVENTION Compositionally the ceramic igniter of thepresent invention consists of to 99.9 percent by weight of alpha siliconcarbide, 0.05 to 0.50 percent by weight of aluminum, 0 to 4 percent byweight silica, 0 to 0.25 percent by weight of iron or iron-basedcompounds, a maximum of lOO parts per million of boron and a minoramount, generally not in excess of 0.25 percent, of miscellaneousimpurities. The composition also contains a very small (on the order of500 ppm) amount of nitrogen which is introduced into the silicon carbideby a doping process which will be described in more detail subsequently.The small amount of aluminum incorporated in the SiC is necessary toraise the high temperature (e.g. l,000C) resistivity of the igniter to alevel on the order of 0.06 to 0.26 ohm centimeters. The boron content ispreferably kept below 50 ppm to maintain reasonably low resistivity atboth low and high temperatures, the low resistivity at room temperaturebeing particularly important from the standpoint of heat up time.

The igniter shape is formed by conventional methods which result in saidigniter having a controlled density of from about 2.60 to 2.70 grams percubic centimeter. This controlled density has the advantage of producinga silicon carbide resistor with a higher resistivity than a more densesilicon carbide. thus facilitating the formation of an igniter with therequired resistance, but with a relatively short electrical path. Theimportance of this latter feature relates to the fact that ignitersgenerally are used in very limited spaces, therefore, must be small insize. The high resistivity of the controlled density igniters of theinvention greatly facilitates this objective. As a result of thecomposition, density, and the processing employed, the resulting siliconcarbide igniter is ideally suited as a fuel igniter for such devices asgas clothes dryers, in that the stringent requirements for such ignitersare easily satisfied by the igniters of the invention. To be acceptablefor such end uses, the igniter must have sufficient mechanical strengthto resist severe physical forces; the present igniter will withstand awhipping type force of at least gs. Such an igniter must also be able toattain a temperature of about l,000C in less than 20 seconds whiledrawing a maximum of 6 amps at 132 volts, and in less than 60 seconds atan input of 80 volts; the present igniter easily satisfies theserequrements by virtue of a room temperature resistivity of 0.l0 to L70ohm centimeters, and a resistivity at approximately l,000C of 0.06 to0.26 ohm centimeter. Its overall physical dimensions for gas firedclothes dryers and ranges is from 2.125 to 2.625 inches in length, withan effective cross-section of from 0.012 to 0.072 square inch. Finally,the present igniter has an inherent ability to withstand at least200,000 heat-up and cool-down cycles. This is unexpected in view of therelatively low density of the igniter, but it is believed that thisresults from a combination of chemical composition, processingconditions involved in the fabrication of said igniter, and the highpercentage of the heating area which radiates directly to theenvironment. By the expression area which radiates directly to theenvironment we mean hot area that does not see" other hot areas. Thusthe inside surface of a cylindrical heating element would see other hotportions of the inside surface (or a hot support element) and would notbe considered as radiating directly to the environment." The hot" areaof the igniter of FIG. I is the surface of that part of the element ofsmallest cross-section, that is the portion of 8a, 8b, a, and 10b ofminimum cross-section. In FIG. 2, about 55% of the surface of the hotarea is outside" surface. To keep the outside surface above 50%, thethickness of the igniter should not be greater than twice the width ofthe legs. From the design of FIG. 3, the outside area will always begreater than 50%.

The present igniter is monolithic and self-supporting, needing nosupporting device such as that required for the successful utilizationof the silicon carbide igniter of US. Pat. Nos. 3,372,305 and 3,467,812.This results from the relatively great thickness, i.e., cross-sectionalarea of the present igniters as set forth above. The most desirableconfiguration is that of a leg having a hairpin shape including terminalconnecting ends, because this shape presents at least 50 percent of thesurface area of the hot zone of the igniter to the surroundingenvironment. With a high percentage of the heating area radiatingoutward, there is less tendency for hot spots to develop. Thischaracteristic, plus the relatively large cross-section, minimizespremature burn out. It is even more desirable that the igniter be madeof two legs of hairpin configuration to maximize the igniters ability toquickly ignite a fuel exposed thereto.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal view of thelargest surface area of the igniter of the present invention.

FIG. 2 is a sectional view of the igniter of FIG. 1.

FIG. 3 is a longitudinal view of the largest surface area of anotherembodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred physicalconfiguration of the instant igniter is shown in FIGS. 1 and 2.Referring to FIG. 1 the wing shaped elements 4 and 6 are terminalconnect ing ends. Coextensive with the terminal connecting ends and witheach other are two hairpin shaped legs 8 and I0. The double hairpinconfiguration is completed by the approximately centrally located slot12 which traverses from the end of the igniter opposite the terminalconnecting ends towards said ends but stopping substantially shortthereof; and a slot in each leg 8 and 10 identified as 14 and 16respectively in FIG. 1. The electrical path begins at the terminalconnecting ends 4 and 6 and traverses the legs through a substantialpart of their length, forming two elements 8a, 8b and Illa and 10b foreach leg. In the slots I4 and 16 at the terminal connecting ends thereofit is desirable, although not absolutely necessary, to include a portionof an electrically insulating cement such as a commercially availablealumina based refractory cement. This is shown as small dabs l8 and 20.Larger quantities of refractory cement may be used if desired. Withoutthe portion of cement so located in slots 14 and 16 there is the dangerof shorting out or breaking of the igniter should any force be exertedon the terminal connecting ends 4 and 6 so as to force said ends towardone another. The ends or tips 22 and 24 of legs 8 and 10 respectivelyhave a larger cross-section than the crosssection of their individualelements 80, 8b, 10a and 10b. This larger cross-sectional area of theseends causes them to remain relatively cool and causes concentration ofthe hot zone of the igniter in those portions of the two legs in betweenthese ends 22 and 24 and the terminal connecting ends 4 and 6. Thisconfiguration exposes, for direct radiation to its environment, at least50% of the total surface area of the igniters hot zone. In calculatingthe area of hot zone which radiates directly to the environment in FIGS.1 and 2 the upper and lower surfaces (those parallel to the plane of thedrawing) and the outer boundaries of the element would be considered asthe applicable areas. The surfaces of the element defining the slotswould not be so considered since they can radiate directly to theirhotfacing surfaces.

In a preferred form for gas dryers the present igniter is from 2.125 to2.625 inches in length, with the end 22 and 24 of the legs 8 and 10 eachhaving an essentially rectangular cross-sectional area of from 0.020 to0.039 square inch. Elements 8a, 8b, 10a and 10b of legs 8 and 10 eachpreferably have a cross-section of from 0.009 to 0.014 square inch, theslots forming said elements are preferably from 0.033 to 0.080 inchwide. There are many possible variants on the basic configuration of thepresent igniter, one such being that shown in Fig. 3 which has terminalconnecting ends 26 and 28 and a single hairpin shaped leg 30 comprisedof elements 30a and 30b, slot 32. Insulating cement 34, is includedbetween the terminal connecting ends 26 and 28. The end 36 has aslightly larger cross-sectional area than elements 30a and 30b of leg30.

In one method of forming the present igniters a casting slip is preparedhaving the preferred composition of 97 to 99.9% by weight of a 50%mixture of high purity 3.0 micron silicon carbide and IOOF siliconcarbide, and 0.05 to 0.30% by weight of A1 0 The preparation of theslip, and the casting thereof into plaster molds, is taught in US. Pat.No. 2,964,823. The mold cavity has a cross-sectional configuration anddimensions corresponding to the outline of the igniter shown in FIG. 1or FIG. 3. The length of the mold cavity is 12 inches although obviouslysaid dimension could be longer or shorter if desired. The green billetthus cast is allowed to stand in the mold for 10 to 15 minutes afterwhich it is removed and air dried for 8 to l6 hours at to C. Tofacilitate slicing of the billet into igniter blanks, the billet isimpregnated with a 25% solution in isopropyl alcohol of a mixture of I00parts by weight of Fapreg P3 and 2 parts by weight of Activator, bothmaterials manufactured and sold by Quaker Oats Company. Otherpolymerizable organic material may also be used in place of theforegoing. The impregnation is carried out by immersion of the greenbillet in the solu tion. The saturated billet is heat treated at about95C for at least l2 hours after which temperature is raised to about190C and held there for 2 hours. The billet is then allowed to cool.

The billet is sliced into igniter blanks preferably about 0.135 inch inthickness. The slicing is best accomplished with a diamond cut-offwheel. The three slots 12, 14 and 16 of FIG. 1 are cut into the blanks,again with a diamond cut-off wheel.

The green igniters are placed in a graphite holder and fired at 2,200 to2,450C in a reducing atmosphere for A to 4 hours. The fired igniters aresubjected to a subsequent firing, in nitrogen, at 1,500 to 2,000C for to180 minutes, maintaining the nitrogen environment until the temperaturein the furnace has dropped to 800C.

The terminal connecting ends 4 and 6 in FIG. I are then coated with ametal, preferably aluminum or an aluminum alloy. This may beaccomplished by any known method such as dipping of the ends into moltenmetal or flame spraying. The ends should also be sandblasted lightlyprior to applying the metal coating.

The final step in the fabrication of the present igniter is the placingof the refractory, electrically insulating cement, l8 and 19 in FIG. I.The cement may be essentially and refractory, electrically insulatingcement but the preferred cement is the high alumina type. The quantityof cement required, for the purposes stated above, is small e.g. anamount of cement to fill the slots 14 and 16 of FIG. 1, approximately Ainch in from the far edge ofthe terminal connecting ends. The slots maybe filled further, if desired.

For optimum performance the igniter should be composed of from 97 to99.9% by weight of polycrystalline silicon carbide, 0.1 to 0.3% byweight of aluminum added as aluminum oxide in the original mixture, lessthan 50 parts per million of boron, and not more than 0.20% ofmiscellaneous impurities. It would also appear that an indeterminateamount ofnitrogen must be introduced into the structure by subjectingthe initial green igniters first to a standard non-oxidizing type offiring step at about 2,200C or above, followed by firing in a nitrogenatmosphere at l,500 to 2,000C. Attempts to combine these two steps intoone fail to affect the desired electrical properties in the finaligniter. This is believed to be due to the different rates of Ndiffusion into the SiC crystals at the two different temperatures. WhenN, is present during the initial high temperature firing (2,200 to2,400C) it diffuses in sufficient quantities into the body of the SiC sothat bulk SiC has a low resistivity both at room and high temperaturesthus providing too much current flow at the high temperature (over 6amps at 132 volts). It is believed that when the igniter is fired innitrogen at the lower temperature (l,500 to 2,000C) a small butsufficient amount of nitrogen diffuses into the surface of the finesilicon carbide particles, which bridge the larger particles, to lowerthe room temperature resistivity of the igniter without significantlyaffecting the high temperature resistivity. As a result this added Nlowers the igniter response time, e.g., the time for the igniter toreach the desired fuel ignition temperature.

Some prior art gas and liquid fuel igniters have the inherentshortcoming of room temperature resistivities that are too high, andelevated temperature resistivities that are too low for the mosteffective and efficient op eration. The igniter of the present inventionis free of this problem having a preferred resistivity at roomtemperature of from 0.15 to 0.5 ohm centimeter and at about 1,000C of atleast 0.1 ohm centimeter, resulting in a response time at volts of 10 to60 seconds to attain approximately 1,000C.

This unique set of resistivities results primarily from the combinationof the introduction of the prescribed amount of aluminum into thecrystal lattice of the silicon carbide, and the post-firing nitrogentreatment which introduces a relatively high percentage of nitrogen intothe crystal lattice of the finer silicon carbide grains. This sametreatment (it is believed) introduces only a very small percentage ofnitrogen into the crystal lattice of the larger SiC crystals. The effectof the presence of aluminum is to increase the resistivity of the body,both at room temperature and at elevated tern perature; the latter isdesirable but the former is not. The nitrogen treatment subsequent tothe initial firing reverses or compensates for the undesirable increasein the room temperature resistivity caused by the introduction of thealuminum, i.e., the nitrogen decreases the room temperature resistivity.The resulting igniter thus has a heretofore unknown combination of arelatively high elevated temperature resistivity and a low roomtemperature resistivity.

The oxygen content of the finished igniter is between about 0.04 to0.1%. After use the oxygen content will increase substantially due tosurface oxidation of the silicon carbide grains. This additional oxygenis not detrimental so long, as it is on the surface of the fired igniterand not between the SiC grains of the igniter where it would introduce ahigh resistance. In some cases it may be desirable to oxidize theigniters prior to sale or to apply an oxide coating on the finishedigniter; these techniques are known in the art.

Where the expression percent or 7r is used in the specification andclaims it is intended to mean weight percent unless clearly stated tohave some other meaning.

What is claimed is:

l. A ceramic resistance igniter, comprised of a pair of terminalconnecting ends and a hot-zone extending therefrom and having acomposition consisting essentially of from to 99.9% by weight of siliconcarbide, 0.05 to 0.50% by weight of aluminum, 0 to 4% by weight ofsilicon oxide, 0 to 0.25% by weight if iron or compounds thereof, amaximum of parts per million of boron, and up to 0.25% by weight ofmiscellaneous impurities, said composition having been sintered and thenexposed to a nitrogen atmosphere at a temperature of from l,500C to2,000C for 15 to 180 minutes.

2. The ceramic resistance igniter of claim 1 having electricalcharacteristics such that said igniter draws a maximum of 6 amps at 132volts and has an impact resistance of at least gs.

3. The ceramic resistance igniter of claim 2 having a response time at80 volts of 60 seconds or less to attain 1,000C and an operational lifeof at least 200,000 cycles.

4. A monolithic ceramic resistance igniter, comprised of a pair ofterminal connecting ends and a hotzone extending therefrom and having acomposition consisting essentially of from 95 to 99.9% by weight ofpolycrystalline silicon carbide, 0 to 4% by weight of silicon oxide, 0to 0.25% by weight of iron or compounds thereof, 0 to 50 parts permillion of boron, and up to 0.25% by weight of miscellaneous impurities;said silicon carbide containing from 0.05 to 0.50% by weight of aluminumin the crystal lattice thereof and nitrogen being introduced into saidcrystal lattice by subjecting said composition to an atmosphere ofnitrogen at a temperature of from 1,500C to 2,000C for 15 to 180minutes.

5. The ceramic resistance igniter of claim 4 having a room temperatureresistivity of 0. 10 to 1.70 ohm centimeters and a resistivity at l,000Cot0.06 to 026 ohm centimeter.

6. A sintered ceramic resistance igniter, comprised of a pair ofterminal connecting ends and a hot-zone extending therefrom and having acomposition consisting essentially of from 97 to 99.9% by weight ofpolycrystalline silicon carbide, 0.1 to 0.3% by weight of aluminumcontained in the crystal lattice of said silicon carbide, to 100 partsper million of boron, and from 0 to 0.2% by weight of miscellaneousimpurities, said composition having been doped with nitrogen by heatingat 1,500C to 2,000C for 15 to 180 minutes; said ceramic igniter having aroom temperature resistivity of 0.15 to 0.5 ohm centimeter, aresistivity at l,800F of at least 0.1 ohm centimeter. a response time ofto 60 seconds to attain 1,000C, an operational life of at least 200,000cycles, an impact resistance of at least 125 gs, and the furtherproperty that said igniter draws a maximum of 6 amps at 132 volts.

7. A sintered ceramic resistance igniter, comprised of a pair ofterminal connecting ends and a hot-zone extending therefrom and having acomposition consisting essentially of from 95 to 99.9% by weight ofpolycrystalline alpha silicon carbide, 0 to 4% by weight of siliconoxide, 0 to 100 parts per million of boron, 0.05 to 0.5% by weight ofaluminum, said composition having first been preformed and fired at2,250C to 2,450C in an inert atmosphere followed by firing in a nitrogenatmosphere at from 1,500C to 2,000C for to 180 minutes; said igniterhaving a density of 2.60 to 2.70 gms/cc. and having resistivity at roomtemperature of from 0.10 to 1.70 ohm centimeters and at l,000C of from0.06 to 0.26 ohm centimeter.

8. A sintered ceramic resistance igniter, comprised of a pair ofterminal connecting ends and a hot-zone extending therefrom and having acomposition consisting essentially of from to 99.9% by weight ofpolycrystalline alpha silicon carbide, 0 to 4% by weight of siliconoxide, 0 to parts per million of boron, 0.05 to 0.5% by weight ofaluminum, said igniter having a density of 2.60 to 2.70 gms/cc, havingresistivity at room temperature of from 0.10 to 1.70 ohm centimeters andat 1,000C of from 0.06 to 0.26 ohm centimeter, and having at least 50%of the surface area of the hot zone of the igniter radiating directly tothe environment.

9. A ceramic resistance igniter, comprised of a pair of terminalconnecting ends and a hot-zone extending therefrom and having acomposition consisting of from 95 to 99.9% by weight of silicon carbide,0.05 to 0.5% by weight of aluminum, 0.04 to 0.1% by weight of oxygen, 0to 4% by weight of silicon oxide, 0 to 0.25% by weight of iron orcompounds thereof, a maximum of 100 parts per million of boron, saidcomposition having been exposed to a nitrogen atmosphere at atemperature of from 1,500C to 2,000C for 15 to 180 minutes.

10. The ceramic resistance igniter of claim 9 wherein said ends havingbeen treated with an aluminum alloy.

11. A monolithic ceramic resistance igniter having a flat elongatedconfiguration essentially rectangular in cross-section, includingterminal connecting means at one end, a hot zone extending therefromcomprised of at least one leg having a hairpin shape, where the end ofsaid leg opposite the terminal connecting ends has a greatercross-section than the cross-section of the individual elements makingup said hairpin shaped leg, and having at least 50% of the surface areaof said hot zone radiating directly to the environment.

12. The monolithic ceramic resistance igniter of claim 11 comprised ofpolycrystalline silicon carbide and consisting of two interconnectedhairpin shaped legs, the overall length of said igniter being from 2.125to 2.625 inches, the ends of said legs opposite the terminal connectingends having a cross-sectional area of from 0.013 to 0.049 square inch,the elements of said hairpin shaped legs having a cross-section of from0.006 to 0.018 square inch, and the width of the slots separating saidelements being from 0.012 to 0.080

inch.

1. A CERAMIC RESISTANCE IGNITER, COMPRISED OF A PAIR OF TERMINALCONNECTING ENDS AND A HOT-ZONE EXTENDING THEREFROM AND HAVING ACOMPOSITION CONSISTING ESSENTIALLY OF FROM 95 TO 99.9% BY WEIGHT OFSILICON CARBIDE, 0.05 TO 0.50% BY WEIGHT OF ALUMINUM, 0 TO 4% BY WEIGHTOF SILICON OXIDE, 0 TO 0.25% BY WEIGHT OF IRON OR COMPOUNDS THEREOF, AMAXIMUM OF 100 PARTS PER MILLION OF BORON, AD UP TO 0.25% BY WEIGHT OFMISCELLANEOUS IMPURITIES, SAID COMPOSITION HAVING BEEN SINTERED AND THENEXPOSED TO A NITROGEN ATMOSPHERE AT A TEMPERATURE OF FROM 1,500*C TO2,000*C FOR 15 TO 180 MINUTES.
 2. The ceramic resistance igniter ofclaim 1 having electrical characteristics such that said igniter draws amaximum of 6 amps at 132 volts and has an impact resistance of at least125 g''s.
 3. The ceramic resistance igniter of claim 2 having a responsetime at 80 volts of 60 seconds or less to attain 1,000*C and anoperational life of at least 200,000 cycles.
 4. A monolithic ceramicresistance igniter, comprised of a pair of terminal connecting ends anda hot-zone extending therefrom and having a composition consistingessentially of from 95 to 99.9% by weight of polycrystalline siliconcarbide, 0 to 4% by weight of silicon oxide, 0 to 0.25% by weight ofiron or compounds thereof, 0 to 50 parts per million of boron, and up to0.25% by weight of miscellaneous impurities; said silicon carbidecontaining from 0.05 to 0.50% by weight of aluminum in the crystallattice thereof and nitrogen being introduced into said crystal latticeby subjecting said composition to an atmosphere of nitrogen at atemperature of from 1,500*C to 2,000*C for 15 to 180 minutes.
 5. Theceramic resistance igniter of claim 4 having a room temperatureresistivity of 0.10 to 1.70 ohm centimeters and a resistivity at 1,000*Cof 0.06 to 0.26 ohm centimeter.
 6. A sintered ceramic resistanceigniter, comprised of a pair of terminal connecting ends and a hot-zoneextending therefrom and having a composition consisting essentially offrom 97 to 99.9% by weight of polycrystalline silicon carbide, 0.1 to0.3% by weight of aluminum contained in the crystal lattice of saidsilicon carbide, 0 to 100 parts per million of boron, and from 0 to 0.2%by weight of miscellaneous impurities, said composition having beendoped with nitrogen by heating at 1,500*C to 2,000*C for 15 to 180minutes; said ceramic igniter having a room temperature resistivity of0.15 to 0.5 ohm centimeter, a resistivity at 1,800*F of at least 0.1 ohmcentimeter, a response time of 10 to 60 seconds to attain 1,000*C, anoperational life of at least 200,000 cycles, an impact resistance of atleast 125 g''s, and the further property that said igniter draws amaximum of 6 amps at 132 volts.
 7. A sintered ceramic resistanceigniter, comprised of a pair of terminal connecting ends and a hot-zoneextending therefrom and having a composition consisting essentially offrom 95 to 99.9% by weight of polycrystalline alpha silicon carbide, 0to 4% by weight of silicon oxide, 0 to 100 parts per million of boron,0.05 to 0.5% by weight of aluminum, said composition having first beenpreformed and fired at 2,250*C to 2,450*C in an inert atmospherefollowed by firing in a nitrogen atmosphere at from 1, 500*C to 2,000*Cfor 15 to 180 minutes; said igniter having a density of 2.60 to 2.70gms/cc, and having resistivity at room temperature of from 0.10 to 1.70ohm centimeters and at 1,000*C of from 0.06 to 0.26 ohm centimeter.
 8. Asintered ceramic resistance igniter, comprised of a pair of terminalconnecting ends and a hot-zone extending therefrom and having acomposition consisting essentially of from 95 to 99.9% by weight ofpolycrystalline alpha silicon carbide, 0 to 4% by weight of siliconoxide, 0 to 100 parts per million of boron, 0.05 to 0.5% by weight ofaluminum, said igniter having a density of 2.60 to 2.70 gms/cc, havingresistivity at room temperature of from 0.10 to 1.70 ohm centimeters andat 1,000*C of from 0.06 to 0.26 ohm centimeter, and having at least 50%of the surface area of the hot zone of the igniter radiating directly tothe environment.
 9. A ceramic resistance igniter, comprised of a pair ofterminal connecting ends and a hot-zone extending therefrom and having acomposition consisting of from 95 to 99.9% by weight of silicon carbide,0.05 to 0.5% by weight of aluminum, 0.04 to 0.1% by weight of oxygen, 0to 4% by weight of silicon oxide, 0 to 0.25% by weight of iron orcompounds thereof, a maximum of 100 parts per million of boron, saidcomposition having been exposed to a nitrogen atmosphere at atemperature of from 1,500*C to 2,000*C for 15 to 180 minutes.
 10. Theceramic resistance igniter of claim 9 wherein said ends having beentreated with an aluminum alloy.
 11. A monolithic ceramic resistanceigniter having a flat elongated configuration essentially rectangular incross-section, Including terminal connecting means at one end, a hotzone extending therefrom comprised of at least one leg having a hairpinshape, where the end of said leg opposite the terminal connecting endshas a greater cross-section than the cross-section of the individualelements making up said hairpin shaped leg, and having at least 50% ofthe surface area of said hot zone radiating directly to the environment.12. The monolithic ceramic resistance igniter of claim 11 comprised ofpolycrystalline silicon carbide and consisting of two interconnectedhairpin shaped legs, the overall length of said igniter being from 2.125to 2.625 inches, the ends of said legs opposite the terminal connectingends having a cross-sectional area of from 0.013 to 0.049 square inch,the elements of said hairpin shaped legs having a cross-section of from0.006 to 0.018 square inch, and the width of the slots separating saidelements being from 0.012 to 0.080 inch.